1. Field of the Invention
The present invention relates to a radar system that detects a target by transmitting and receiving electric waves generated by performing frequency modulation on continuous waves.
2. Description of the Related Art
An FM-CW radar system that detects a target by transmitting and receiving electric waves generated by performing frequency modulation (FM) on continuous waves (CW) transmits a transmitting signal in which a gradually-frequency-rising up-modulating interval and a gradually-frequency-dropping down-modulating interval change temporally repeatedly in a triangular waveform, and receives a reception signal including reflected signals from the target, whereby a relative distance to the target and its relative speed are determined based on the frequency spectrum of a beat signal which is a signal representing a frequency difference between the transmitting signal and the reception signal. Also, by performing the above-described operation for one beam directed to a predetermined direction or bearing, and sequentially changing the beam bearing, detection of targets spread in a predetermined bearing-angle range is performed.
When there is a single target, respectively in both the up-modulating interval and the down-modulating interval, a single peak appears in the frequency spectrum of a beat signal based on reflected waves from the target. Thus, based on a frequency at a peak in the frequency (hereinafter referred to as the xe2x80x9cupbeat frequencyxe2x80x9d) of the beat signal in the up-modulating interval and another frequency at another peak in the frequency (hereinafter referred to as the xe2x80x9cdownbeat frequencyxe2x80x9d) of the beat signal in the down-modulating interval, a relative distance to the target and its relative speed is determined.
Conversely, when there are a plurality of targets in a search range, for a single beam, many peaks appear in a frequency spectrum in both the up-modulating interval and the down-modulating interval. This causes a possibility that an error may occur in pairing a plurality of upbeat frequencies and a plurality of downbeat frequencies.
Accordingly, in Japanese Unexamined Patent Application Publication No. 2000-65921, a radar system is disclosed in which peaks at identical frequencies which appear in frequency spectrum are grouped and those having equal major beam bearings are paired.
In connection therewith, research conducted by the present inventors yielded the discovery of the following problems.
Specifically, appropriate grouping cannot be performed for targets having large relative speeds in a state in which a vehicle having a radar system installed therein travels at a high speed, such as a roadside object and an oncoming vehicle, because the difference between the upbeat frequency and the downbeat frequency is large.
In addition, when there is heavy traffic, or when there are many roadside objects, many groups having equal major beam bearings are detected, so that pairing with good precision is impossible.
Also, peaks at identical frequencies may appear in the frequency spectrum as an upbeat frequency and a downbeat frequency due to effects of noise electric waves coming from the outside of the radar system and noise generated inside the radar system. As a result, a relative distance to an actually nonexistent target and its relative speed are determined and that pairing is affected.
In order to overcome the problems described above, preferred embodiments of the present invention provide a radar system in which appropriate grouping is performed even for a target having a large relative speed and in which accurate pairing can be performed even if a plurality of objects exist.
Also, preferred embodiments of the present invention provide a radar system in which a problem caused by external noise and in-apparatus noise is eliminated.
According to a preferred embodiment of the present invention, a radar system includes a transmitting/receiving unit for transmitting a transmitting signal changing in a triangular waveform so that a frequency-gradually-rising up-modulating interval and a frequency-gradually-dropping down-modulating interval are temporally repeated and for receiving a reception signal including a reflected signal from a target, a scanning unit for changing the beam bearing of the transmitting signal for predetermined bearing angles, a frequency-analyzing unit for finding data on the frequency spectrum of a beat signal representing a frequency difference between the transmitting signal and the reception signal, a detecting unit for detecting, based on the frequency of the beat signal in the up-modulating interval and the frequency of the beat signal in the down-modulating interval, at least one of a relative distance to the target and a relative speed thereof, and a checking unit in which, peak groups each including consecutive peaks appearing in the frequency spectrum in the beam bearings within a predetermined frequency difference are regarded as being caused by waves reflected by a single target, and the unit performs checking, in pairs, of each peak group in the up-modulating interval and each peak group in the down-modulating interval.
As described above, groups of consecutive peaks in the beam bearings within a predetermined frequency difference are regarded as being caused by reflected waves from a single target, whereby appropriate grouping can be performed even for a target having a large relative speed. In addition, even if there are many objects, one peak group can be detected for one target, thus preventing a pairing error from occurring.
Preferably, the checking is performed only on each group in which the number of the peaks which are consecutive in the beam bearings is a predetermined number or greater.
This prevents noise components having a short duration, such as random noise, from being mistakenly extracted as peak groups.
A peak group in which the peaks consecutively appear in all of the beam bearings and the signal intensity of the peaks may be substantially constant are regarded as a specific group, and the checking may be performed excluding the specific group.
This prevents a signal component other than a beat frequency caused by reflected waves from the actual target from being used for checking. This prevents a pairing error from occurring.
When the specific group appears in both the up-modulating interval and the down-modulating interval, the specific group may be excluded.
When the specific group repeatedly appears in accordance with the repetition of changing of beam bearings, the specific group may be excluded.
The scanning unit can change the speed of changing the beam bearing, and/or the transmitting/receiving unit can change the speed of repeating the up-modulating interval and the down-modulating interval, and when the frequency of the specific group is substantially constant despite a change in the speed of changing the beam bearing or a change in the speed of repeating the up-modulating interval and the down-modulating interval, the specific group may be excluded.
As described above, by excluding a continuously appearing noise component, an undesired influence on pairing is prevented.
Among the peak groups, the checking may be performed on two peak groups having substantially equal central bearings.
This facilitates checking for many targets, even if they are spread out in beam bearings.
Among the peak groups, the checking may be performed on two peak groups having substantially equal signal intensities. This ensures checking for peak groups caused by a single target.
The radar system may be installed in a movable object and preferably includes unit for capturing information on the movement speed of the movable object, and a possible frequency range generated by combining the peak in the up-modulating interval and the peak in the down-modulating interval may be set based on the movement speed, and the checking is performed for the frequency range.
As described above, by limiting the number of ones to be checked by avoiding normally impossible combinations of relative distances and relative speeds, the time required for the checking is shortened, and many targets can simultaneously be captured. In addition, a possibility of mis-pairing occurring is minimized.
Also, correlations between the signal-intensity profiles in the bearings of the peak groups may be determined, and the checking may be performed so that a higher priority is given to a peak group having a higher correlation.
This increases precision of pairing of peak groups caused by a single target.
Also, according to preferred embodiments of the present invention, a possible frequency range generated by combining the peak in the up-modulating interval and the peak in the down-modulating interval is set based on the movement speed of a movable object having a radar system installed therein, and the checking is performed for the frequency range, whereby the time required for the checking is shortened, and many targets can simultaneously be captured. In addition, a possibility of mis-pairing occurring is minimized.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.